1. FIELD OF THE INVENTION
This invention relates to a speed control device for an electric sewing machine, and more particularly to a device for controling speed acceleration and reduction of a machine motor of a sewing machine.
In the electric sewing machine having a circuit for feeding back a present value to a designated rotation number, when a pedal controller is stepped rapidly, the sewing machine is rotated at high speed instantaneously, which gives a machine operator an uneasy feeling.
Due to time lag in a feedback system, the rotating number is overshot, and the rotation is higher or lower than the designated number for a while.
FIG. 1 shows one example of a controlling block diagram of a conventional electric sewing machine.
An analog voltage from a controller (CONT) is input into an A/D converted (A/D) at IN thereof, and an input is A/D converted. A converted digital data is an address data in a speed setting data memory (VDATA) and is given as speed setting data to a calculator (ACC), and compared with an actual speed data issued from a speed detector (VACC), thereby to control conduction time of a semi-conductor element installed in a machine motor drive circuit (DV), so that a machine motor (M) is controlled to have rotation number in response to an input of an input terminal (IN) of the A/D converter (A/D).
Therefore, in order to settle the above mentioned inconvenience, a delay circuit between a resistor (R) and a condensor (C) is provided between a slide terminal (S) of a variable resistor (VR) within the controller (CONT) and an input terminal (IN) of the A/D converter (A/D), whereby the voltage of the input terminal (IN) of the A/D converter (A/D) is not rapidly increased if the pedal controller (CONT) is rapidly stepped. A diode (D) is provided for instantly changing a level of the input terminal (IN) of the A/D converter (A/D) into OV, when the controller (CONT) is released.
Even if the controller (CONT) is suddenly stepped, a time constant to be determined by a value between the resistor (R) and the condensor (C) realizes smooth speed acceleration, so that the machine operator does not feel uneasy or the rotation number is not overshot.
The time constant of the delay circuit is determined by the value of the resistor (R) and the condensor (C). If the value is changed due to dispersions per each of products, changes in temperatures or variations in passing time, the machine motor (M) does not run at the designed accelerating curve.